Field experiments were conducted in 1981 and 1982 to
develop a method for using remotely measured leaf canopy-air
temperature differences (stress-degree-days (SDD)) to
schedule irrigations and to evaluate the effects of
differential irrigation on snap bean cultivars, 'Oregon
1604' and 'Galamor'. Effects on soil water potential
(SWP), leaf water potential (Ψ), leaf osmotic potential
(fir ), leaf turgor potential (Ψp), leaf transpiration,
leaf diffusive resistance, plant growth and development,
yield components, and pod quality factors were measured.
To schedule irrigations, various levels of positive
SDD's, measured daily at midday, were allowed to
accumulate between irrigations. When air vapor pressure
deficits (VPD) were small, water stressed plants had
positive. SDD's and well watered plants had negative
SDD's. When VPD's were large, SDD's were always negative
(i.e., canopy temperatures were cooler than air
temperature). At these high VPD's, temperature
differences between leaves of well watered and stressed
plants were as much as 5-7 C. A model is presented in
which SDD's can be adjusted based on the expected SDD of
well watered plants at a given VPD. With this
modification, irrigations can be adequately scheduled
using only canopy temperatures, air temperatures, and
relative humidity measurements.
In both seasons, yield was strongly related to
average SWP at 0-45 cm depth. In 1981, evaporative
demand on the crop was much higher than in 1982. As a
result, yields in 1982 were at least 5 t/ha greater at a
given SWP than in 1981. Under water stress, total leaf
area per plant was reduced more by a decrease in area per
leaf than by a reduction in leaf number.
Leaf water potential was not consistent with level
of plant water stress. Although some osmotic adjustment
occurred in stressed plants, Ψp was still generally lower
than in well watered plants throughout the day. As Ψ
decreased from early morning through midday,
transpiration rates increased due to increasing
evaporative demand. Leaf diffusive resistance also
increased with decreasing V, but a "threshold value" for
stomatal closure was not demonstrated.